Degradation of a fuel cell stack results from oxidation of catalyst support materials which reduces fuel cell stack performance and shortens the life of the fuel cell stack. High temperatures and high potentials in the fuel cells exacerbate the degradation. A known method to reduce the time that the cells are at high potentials and high temperatures is to remove the oxygen from the air side volume of the fuel cell stack (including the oxidant flow fields in the cells, inlet and exit manifolds, plumbing, etc.), and to stabilize hydrogen levels in both the cathode and anode flow fields, which prevents high voltages in the cells. The hydrogen stabilization process is sometimes referred to as “hydrogen on”.
Typical hydrogen stabilization is accomplished in a shutdown procedure which includes (a) blocking the input of fresh air into the stack, (b) removing the service load and connecting the stack to a resistive auxiliary load, (c) performing cathode recycle by conducting cathode exhaust to the inlet of the cathode blower, which remains on, while continuing to provide fresh hydrogen to the cells, with fuel recycle on, and with fuel purge blocked; this stage of the procedure continues until sufficient hydrogen has been provided to react with all of the residual oxygen in the air-side volume, (d) closing off the inlet of fresh hydrogen to the fuel cells while continuing to provide cathode and fuel recycling.
After the supply of fresh hydrogen has been isolated from the system, the consumption of residual oxygen with hydrogen and condensation of water vapor, due to cooling of the system, creates a vacuum which, if not otherwise relieved, causes air to be sucked into the stack from wherever there are leak paths in the system. To relieve this vacuum in a controlled manner, it is known to vent the anode and/or the cathode to ambient, at least a small amount. In the case of the cathode, any gas flow through its exhaust system will present unwanted oxygen to the fuel cells. In the case of the anode, the small amount of suction may only result in drawing some of the previously expelled anode exhaust, consisting mainly of hydrogen and nitrogen, back into the cells, which would be harmless. But if the suction is too large, then air will follow the previous anode exhaust into the fuel cells and exacerbate the problem of oxygen causing high voltages and oxidation of the catalyst support material.